1. Field of the Invention
The present invention relates to a positive active material for a rechargeable lithium battery and a method of preparing the same, and, more particularly, to a positive active material for a rechargeable lithium battery exhibiting good electrochemical properties such as cycle life, high discharge potential, high power density and improved thermal stability characteristics.
2. Description of the Related Art
Rechargeable lithium batteries have high average discharge potential of about 3.7V and are 4V-grade batteries. The rechargeable lithium batteries are widely used for cellular phones, notebook computers, or camcorders, which are also known as “the 3Cs”, and are main components in the digital world.
The rechargeable lithium batteries use a material from or into which lithium ions are deintercalated or intercalated as positive and negative active materials. For an electrolyte, an organic solvent or polymer is used. Rechargeable lithium batteries produce electric energy as a result of changes in the chemical potentials of the active materials during the intercalation and deintercalation reactions of lithium ions.
For the negative active material in a rechargeable lithium battery, metallic lithium has been used in the early days of development. Recently, however, carbon materials, which intercalate lithium ions reversibly, are extensively used instead of the metallic lithium due to problems of high reactivity toward electrolyte and dendrite formation of the metallic lithium. With the use of carbon-based active materials, the potential safety problems that are present in batteries with the metallic lithium can be prevented while achieving relatively higher energy density, as well as much improved cycle life. In particular, boron is added to carbonaceous materials to produce boron-coated graphite (BOC) in order to increase the capacity of the carbonaceous materials.
For the positive active material in the rechargeable lithium battery, chalcogenide compounds into or from which lithium ions are intercalated or deintercalated are used. Typical examples include LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2(0<X<1) or LiMnO2. Manganese-based materials such as LiMn2O4 or LiMnO2 are the easiest to prepare, are less expensive than the other materials, and are environmentally friendly. However, manganese-based materials have a low capacity. LiNiO2 is inexpensive and has a high charge capacity, but is difficult to produce. LiCoO2 is relatively expensive, but widely used as it has good electrical conductivity and high battery voltage. Most rechargeable lithium batteries (about at least 95%) employ LiCoO2.
Although LiCoO2 exhibits good cycle life characteristics and good flat discharge profiles, there are still demands to improve electrochemical properties such as good cycle life and high power density.
One way to satisfy such a demand is to substitute a part of the Co from LiCoO2 with other metals. Sony prepares LixCo1-yMyO2 by doping about 1 to 5 percent by weight of Al2O3 into LiCoO2. A&TB (Ashai & Thosiba Battery Co.) prepares a Sn-doped Co-based active material by substituting a part of Co from LiCoO2 with Sn.
Another way is that a lithiated compound is coated with a coating material.
U.S. Pat. No. 5,292,601 discloses LixMO2 (M is at least one element selected from Co, Ni or Mn; x is 0.5 to 1). U.S. Pat. No. 5,705,291 discloses a method in which a coating material is mixed with a lithiated intercalation compound, and the mixture is annealed at 400° C. or more to coat the compound with the coating material. The coating material is selected from boron oxide, boric acid, lithium hydroxide, aluminum oxide, lithium aluminate, lithium metaborate, silicon dioxide, lithium silicate or mixtures thereof.
Japanese Patent Laid-Open No. Hei 9-55210 discloses that lithium nickel-based oxide is coated with alkoxide of Co, Al and Mn and heat-treated to prepare a positive active material. Japanese Patent Laid-Open No. Hei 11-16566 discloses lithium-based oxide coated with a metal and/or an oxide thereof. The metal includes Ti, Sn, Bi, Cu, Si, Ga, W, Zr, B or Mn. Japanese Patent Laid-Open No. 11-185758 discloses coating a surface of lithium manganese oxide with metal oxide by using a co-precipitation procedure and heat-treating the same to prepare a positive active material.
Even though these studies have progressed, there are still demands for improving electrochemical properties such as high capacity, long cycle life, high power density and exhibiting good thermal stability. In addition, much research is being conducted on thermal stability of positive active materials to ensure stability and reliability of batteries under abusive condition such as heat-exposure, firing or overcharging.